The present invention relates in a general manner to the vacuum treatment of any substrate of the kind in which, using a sputtering technique, the deposition of a layer of material coming from a given source of material is carried out, everywhere, in a chamber, on this substrate by subjecting, for example, this source of material, which then serves as a cathode, to a suitable ion bombardment, the result of which is that particles are torn off it and these are then deposited on the substrate.
The invention is aimed more particularly at the case in which the treated substrate is a curved substrate, or even a highly curved substrate, as is the case, at least, in certain spectacle lenses, particularly those of high power.
The expression “curved substrate” should therefore be understood to mean here, more generally, a substrate which, in the manner of a spectacle lens, has a curvature accentuated to a greater or lesser extent in at least one transverse plane.
Thus, as is known, it is usual to apply a vacuum treatment to certain spectacle lenses in order to give them particular properties, for example antireflection properties.
One of the problems to be solved in terms of material is to ensure the complete thickness uniformity desirable for the layer of material thus deposited.
The term “thickness” should be understood to mean here, and throughout the following, the optical thickness of the deposited layer, that is to say the physical thickness of this layer multiplied by the refractive index of the material of which it is composed.
Should good uniformity of this thickness be lacking, any possible residual reflection runs the risk of resulting in undesirable iridescence.
Now, the thickness of the deposited layer of material is inversely proportional to the distance of the spectacle lens, or more generally of the treated substrate, from the corresponding source of material.
When, as is the case here, the substrate is curved, this distance is greater at its periphery than its centre and the thickness of the coating obtained is therefore smaller at this periphery than at its centre.
This thickness difference may become significant when, for practical reasons, the curved substrate is placed a short distance from the source of material, the difference in distance between its periphery and its centre with respect to this source of material then becoming appreciable because of its very curvature.
In practice, this thickness difference may commonly be as much as 20%, depending on the substrates and the chamber employed.